The present invention relates to the accurate control of a d.c.-motor, particularly of the type used in matrix printers.
In many special areas, a.c.-motors with a reducing transmission are used for driving particular components such as carriages. Such a.c.-motors with reducing gear are used, for example, in matrix printers. Of course, an a.c.-motor, such as a single-phase motor, cannot be adequately controlled in regard to speed by means of controlling the magnetic field as it is used, for example, in d.c.-shunt motors. Generally speaking, it must be said that devices and machines having reciprocating components which, in turn, control other elements, are not adequately driven by means of a.c.-motors without mechanical reduction for reasons of adequate lack of speed control. In the case of a matrix printer, a rather high accuracy of the speed is required. A matrix printer usually includes a print head mounted on a carriage and being driven across the platen. The speed of this carriage must be very accurate because it affects the overall appearance of the print. For this reason, a highly accurate speed control is necessary; but an a.c.-motor without mechanical reduction does not provide sufficiently constant speed. In the case of a d.c.-motor, particularly of a d.c.-shunt motor, one provides usually a stator field with a constant voltage so that the magnetic flux is also constant, except for the weakening by armature feedback. Even under load conditions, the d.c.-shunt motor exhibits a rather constant speed. However, in the case of a matrix printer, that constancy, or better that degree of constancy, is insufficient. Even if the motor current is subject to control, the obtained speed constancy is still not adequate.